May sound a little crazy, but has this "no spark" happened using the very same plug? I once ran across a similar situation and no one could find the fault. Since I had tried everything else I pulled the "New plug" and checked continuity between the top, where the wire connects , and the bottom, where the spark jumps to ground and got nothing. Can you believe that?? Hooked up an old plug, hit the starter and walla, nice blue spark. Only seen it once in my life time, but now I know it can happen.